22 January 2013

The universe grows larger, cooler and more complex at astonishing speed until it's a few minutes old. Further change is much slower and less dramatic. Fundamental forces and particles are generated, hydrogen and helium are formed and light is released.

The first few minutes of the universe's existence see a huge increase in volume and a dramatic reduction in temperature. Gravity, light, and atomic forces separate from one another. And finally matter comes into existence in the form of hydrogen and helium nuclei and electrons.

More fundamentally we could say that the universe evolves from a simple, evenly distributed beginning and generates greater and greater complexities confined to smaller and smaller volumes as it expands. We'll explore this concept in a later post.

In Part 3 we discussed the beginning but also understood that we can't directly understand or observe it. A good theory of quantum gravity might help, but we don't have one yet.

So how near the beginning can we claim to have any real understanding? The answer is back to 10-43 of a second. If you want to see that as an ordinary fraction you would need to write 1 at the top with 1 followed by 43 zeroes at the bottom. So we understand the universe (in some sense) back to a very, very tiny part of a second.

What exactly do we know from that very early time?

Gravity and inflation - For one thing, gravity and the other fundamental forces may have all been of equal strength at first, with gravity separating out at 10-43 seconds. There is theoretical support for this. After gravity separated to become the very mild force it is today, the universe entered a time of extremely rapid expansion known as inflation.

This is not just something scientists have dreamed up; the observed properties of the universe can only be explained by such a rapid inflation during which it became unimaginably larger in a tiny, tiny fraction of a second. Before inflation the universe was smaller than a sub atomic particle. Inflation ended between 10-33 and 10-32 seconds, but by this time the universe was spacious (perhaps as large as a football) and packed with elementary particles that still exist in our own time - quarks, antiquarks and gluons.

How do we know all this? There are three important things that constrain what is possible.

Theory - Based on what we know of the later universe, theory rules out most hypotheses about the earliest eras. Only an early universe similar to what is described above could have resulted in what we see today.

Cosmology - Observations suggest a great deal. The cosmic background radiation (shown above) and the distribution of galaxy clusters, for example, can only be explained by inflation.

High energy physics experiments - By creating high energies in particle accelerators we can observe the properties and behaviour of particles in a similar state to these early phases of the universe.

Here's one more thing about inflation. If, as many think, our universe began as a quantum fluctuation, then without inflation it would have been the most transient of fluctuations and the universe would have been snuffed out almost immediately while it was still very tiny.

The electroweak epoch - The next stage in the evolution of the universe involved the strong nuclear force separating from the remaining two fundamental forces. Like the earlier events, this too happened at a very early time, around 10-34 seconds. More particles were able to condense out of the soup of energy at this stage, W bosons, Z bosons and Higgs Bosons became common. These are particles that can be generated in our most powerful accelerators today, so we are able to study them and understand them reasonably well.

The universe continued to expand and cool so that by 10-12 seconds bosons could no longer be created. 10-12 seconds is also called a picosecond (one quadrillionth of a second). Lasers with pulses as short as a picosecond are used for cutting and shaping materials, in medicine, and for removing tatoos. It's still a very brief time, but meaningful enough for real life use. Light travels just 0.3 mm in this time.

The quark, hadron and lepton epochs - The universe continued to expand and cool. After it was a picosecond old the electromagnetic and weak forces separated and the universe at this time was full of a dense quark-gluon plasma.

By the end of this epoch at around a microsecond old (one millionth of a second), the universe was cool enough that the quarks could combine to form protons, neutrons and their anti-particles. At an age of about one second the universe was cool enough for particles and anti-particles to annihilate, leaving a small excess of protons and neutrons.

As the universe expanded and cooled further and aged to about ten seconds, electrons and other leptons were also able to annihilate with their anti-particles leaving a small excess of mostly protons, neutrons, electrons, and photons.

Over the next few minutes conditions cooled to a point where atomic nuclei could form, mostly deuterium and helium with a little lithium. At this point the universe contained these nuclei, protons, electrons, and photons. After a further 380 000 years of cooling and expansion the protons and other nuclei combined with the electrons to form hydrogen and helium atoms (and some lithium atoms). This allowed the photons to move freely (the cosmic microwave background radiation), space became transparent and the earliest structures formed. These structures were simply volumes of slightly varying density and temperature. They are the first things we can 'see' directly and are shown in the illustration at the top of the article.

From this point on the universe becomes more and more recognisable to us, albeit still far hotter and denser than today. We will be able to see the rest of the story much more in terms of astronomy.

Questions:

Are you surprised at the amount of change that took place in the first second?